1. Field of the Invention
The present invention relates to a sound volume or sound quality adjusting apparatus, and in particular, to an adjusting apparatus for adjusting sound volume or sound quality with respect to a reproducing signal from a music source by an electronic volume section constructed with analog switches.
2. Brief Description of the Prior Art
The sound volume or sound quality adjusting apparatus of this type is applied to an audio system as shown in FIG. 5. In this figure, a reproducing signal reproduced by music source 1 is supplied to electronic volume section 2 in which the volume or quality of a sound are adjusted, and thereafter is supplied to power amplifier 3. Power amplifier 3 amplifies the reproducing signal having the adjusted sound volume or sound quality, and produces the sound through speaker 4. Electronic volume section 2 is controlled by data signal DT, clock signal CK and strobe signal ST from control section 5 constructed with microcomputers.
FIG. 6 shows an example of the construction of electronic volume section 2 for adjusting the sound volume. In this figure, shift register 21 has an equal number of bits as the number of bits of data required to set the sound volume of electronic volume section 2. The number of bits of latch circuit 22 corresponds to a distance for a minimum damping amount of the electronic volume section 2, such as 81 steps when amounts 0 to -80 dB are damped every -1 dB. Reference numeral 23 is a group of analog switches disposed so that one switch corresponds to one of the bits of latch circuit 22, and reference numeral 24 is a resistance array composed of resistors connected in series to each other to provide the minimum damping amount, and connected to the respective analog switches of analog switch group 23.
In the apparatus constructed above, data signal DT, for setting the operating state of the electronic volume section 2 to a timing of the clock signal CK to achieve synchronization, is serially supplied from control section 5 to electronic volume section 2 and is stored in the shift register 21. Thereafter, strobe signal ST is supplied from control section 5 to electronic volume section 2 and data which was stored in the shift register 21 according to the timing of strobe signal ST, are provided to latch circuit 22. Latch circuit 22 determines the turning-on and turning-off states of the respective analog switches of analog switch group 23 according to the data thus provided.
The respective analog switches of analog switch group 23 are connected to the respective resistors connected in series to each other in resistance array 24 which inputs reproducing signal SIG IN at one end thereof and is connected to ground GND at the other end thereof. Accordingly, a signal at a voltage level based on data signal DT from control section 5 is selected and outputted from signal output SIG OUT through the analog switch group 23. Thus, the reproducing signal which is adjusted in voltage level is outputted from music source 1 from electronic volume section 2 and is then amplified by power amplifier 3, thereafter producing the sound from speaker 4.
FIG. 8 shows an example of construction cf electronic volume section 2 for adjusting the sound quality. In FIG. 8, elements indicated by the numeral 21 to 24 have the same function as shown in FIG. 6. And, the specific explanation are omitted.
In FIG. 8, the reproducing signal SIG IN is applied to a non inverting input terminal of an operational amplifier OP1 through a resistor R.sub.12 and to an end of a resistance array 24. An inverting input terminal of the operational amplifier OP1 is connected to the other end of the resistance array 24 and to an output terminal of the operational amplifier OP1 through a feedback resistor R.sub.11. Analog switches of analog switch group 23 are connected to a parallel resonance circuit (impedance circuit Z) consisting of a capacitor C and an inductance L, and to a resistor R.sub.13 in series. The other end of the resistor R.sub.13 is connected to ground GND. In the sound quality adjusting apparatus using the above described electronic volume section 2, when the most significant switch of the analog switch group 23, that is the closest to the non-inverting input terminal of the operational amplifier OP1, turns on, the impedance circuit Z and resistance R.sub.13 are connected between the non-inverting terminal of the operational amplifier Op1 and ground GND in series. The reproducing signal SIG IN is amplified by a gain A.sub.v1 and outputted to the output terminal of the operational amplifier OP1 as an output signal SIG OUT. The gain A.sub.v1 is expressed by the following equation. ##EQU1## Where the value of the impedance Z is able to be considered to zero when the input signal has the resonance frequency f.sub.0 =1/2 .pi..sqroot.LC calculated by capacitance C and inductance L. And, the value of impedance Z increase at the outer range of the resonance frequency f.sub.0. Accordingly, the sound quality adjusting apparatus obtains an attenuation characteristic in which the resonance frequency f.sub.0 is centered as shown in FIG. 9.
When the least significant switch of the analog switch group 23, that is the closest to the inverting input terminal of the operational amplifier OP1, turns on, the impedance circuit Z and resistance R.sub.13 are connected between the inverting terminal of the operational amplifier OP1 and the earth GND in series. The producing signal SIG IN is amplified by a gain A.sub.v2 and outputted to the output terminal of the operational amplifier OP1 as SIG OUT. The gain A.sub.v2 is expressed by the following equation. ##EQU2## In this case, the sound quality adjusting apparatus obtain a boost characteristic in which the resonance frequency f.sub.0 is centered as shown in FIG. 9.
In the conventional sound volume or sound quality adjusting apparatus mentioned above, the respective analog switches of analog switch group 23 of electronic volume section 2 are switched according to the timing of strobe signal ST outputted from control section 5 so that the switches are operated randomly irrespective of the operating state of music source 1. Further, since the analog switches are normally constructed with C-MOS to be instantly operable, a discontinuous point is generated with respect to the output signals before and after the switching operation of analog switch group 23 as shown in FIG. 7. Therefore, when the sound is produced from speaker 4, the discontinuous portion sounds like a noise.
Discontinuous level E.sub.ou of a signal generated by the operation of electronic volume section 2 can be provided by the following formula when the output levels before and after this switching operation are respectively E.sub.01 and E.sub.02. EQU E.sub.ou =.vertline.E.sub.01 -E.sub.02 .vertline. (1)
For example, when a sine wave is reproduced, EQU E.sub.01 =A sin .omega.t (2) EQU E.sub.02 =B sin .omega.t (3)
and discontinuous level E.sub.ou is provided by the following formula ##EQU3## When .omega.t=90.degree., EQU E.sub.ou max =.vertline.A-B.vertline. (5)
is outputted as a change in level as a result of discontinuation.